Optoelectronic sensor and method for the measurement of distances in accordance with the light transit time principle

ABSTRACT

An optoelectronic sensor ( 10 ) for the measurement of distances or distance changes in accordance with the light transit time principle is provided having a light transmitter ( 12 ) for the transmission of a light signal and having a light receiver ( 16 ) for the reception of the remitted or reflected light signal, wherein an evaluation unit ( 18 ) is provided which is made to fix the transmission time for the light signal relative to a reference time and to digitize the received light signal on a sampling pattern ( 108 ) having a sampling period to determine the reception time of the light signal, and wherein a time base unit ( 38 ) is provided by means of which the transmission time can be shifted relative to the sampling pattern ( 108 ) with a time precision below the sampling period. In this respect, the time base unit ( 38 ) is made to derive the time shift from a first time signal having a first frequency (f1) and a second time signal having a second frequency (f2) different from the first frequency (f1) and to provide any desired time shifts with a time resolution given by the difference period belonging to the first and second frequencies (f1, f2).

The invention relates to an optoelectronic sensor and to a method forthe measurement of distances or distance changes in accordance with thelight transit time principle in accordance with the preamble of claim 1and claim 12 respectively.

The distance of objects can be determined in accordance with the knownlight transit time principle using optoelectronic sensors. For thispurpose, in a time of flight process, a short light pulse is transmittedand the time up to the reception of a remission or reflection of thelight pulse is measured. Alternatively, in a phase process, transmittedlight is amplitude modulated and a phase shift between the transmittedlight and the received light is determined, with the phase shiftlikewise being a measure for the light transit time. Due to eyeprotection regulations, the last named phase modulation processes are inparticular less suitable with low-remitting targets due to the requiredlarge integration times. In the pulse process, the integral power can beprofitably used such that short pulses can be transmitted at a highenergy density and the signal-to-noise-ratio is thus improved for thesingle shot.

The distance measurement can be necessary, for example, in vehiclesafety, in logistics automation or factory automation or in safetyengineering. A distance measurement device based on a reflected lightbeam can in particular respond to a distance change of the reflector orof the reflecting or remitting target. A special use is a reflectionlight barrier in which the distance between the light transmitter andthe reflector is monitored. The light transit time process is also theprinciple according to which distance measuring laser scanners workwhose position vector measures a line or even an area.

If the resolution of the distance measurement ought to reach a precisionin the range of a few tens of millimeters, the light transit time mustbe determined exactly in an order of magnitude of hundreds ofpicoseconds. To achieve a distance resolution of a millimeter, sixpicoseconds have to be covered metrologically. Such a precision can onlybe realized with very cost-intensive electronics with conventionalsystems.

More cost-effective components such as FPGAs (field programmable gatearrays) and other programmable digital logic components typically haveoperating frequencies in the range of some hundreds of MHz. Nanoseconds,but not picoseconds can thus be resolved.

It is therefore necessary for high time precision to achieve precisionsbelow, i.e. better than, these working frequencies. Since it is not aquestion of the absolute position between the transmission time and thesampling of the received signal for a light transit time measurement,higher resolution can be achieved by shifting the transmission time orthe reception time.

A sensor is known from DE 10 2006 048 697 A1 in accordance with thelight transit time principle in which the transmission times relative tothe reception time can be selected by a special implementation of adirect digital synthesis (DDS). It is, however, shown in this respectthat the theoretical resolution of the phase positions are not alwaysreached in practice. Commercially available DDS components which providephase positions in the desired resolution range are simply too expensivefor many applications.

It is known from the still unpublished EP application of the sameapplicant with the file reference 08101432.6 to increase the samplingrate by parallel lines, with each line delaying the signal by anincreasing fraction of the sampling period so that a number of samplingpoints corresponding to the number of lines is produced in each samplingperiod. To compensate a jitter of the delay lines, an additional delayof the received pulse is applied so that so-to-say the delays of thedelay lines are swapped cyclically among one another. This additionaldelay is generated in that two frequencies are moved through, forexample in the ratio 7 to 8, which are displaced with respect to oneanother by a further multiple of a difference period with every furthersampling period. It is, however, not possible with this method to fix atransmission time with the difference period since the two frequenciesonly run through and thus provide at a given time the offset which isjust being applied and not a desired offset. The known method also doesnot have the goal of setting a desired offset, but it is in contrastrather directed to run through the offsets uniformly since only then isthe jitter averaged out.

It is therefore the object of the invention to provide a cost-effectivepossibility for the distance measurement in accordance with the lighttransit time principle with a higher time precision.

This object is satisfied by an optoelectronic sensor in accordance withclaim 1 and by a method for the measurement of distances or distancechanges in accordance with claim 12 respectively.

The solution in accordance with the invention starts from the idea ofutilizing the difference period of two slightly different frequencies.Since a record is kept of the period in which the two frequencies arerespectively located, time intervals can thus be decoupled whoseprecision is given by the difference period.

It is important to note that the resolution is not necessarily the sameas the difference period. This is the case at a ratio of the twofrequencies of n/(n+1) and such a ratio is also preferred. The exampleof other co-prime numbers such as ⅜ shows that the difference frequency5 admittedly fixes the precision, but is not identical with it, sincethe smallest possible offset is also 1 in such a system. The offsetshere do not increase monotonously with time, but all necessary offsetsare equally present after a sorting as in the clearer case n/(n+1). Inthis observation, the units were cut; the consideration does not changeif each number is multiplied by a common base frequency, for example by10 MHz.

The invention thus has the advantage of increasing the resolution givenby the sampling pattern and thus the measurement precision. In thisrespect, simple circuits or software solutions can be use so that thesolution in accordance with the invention is very cost-effective.

The time base unit is preferably made to derive the first frequency andthe second frequency from a master clock which also determines thereference time and to synchronize the first frequency and the secondfrequency to the master clock regularly. Only one stable clock is thusrequired and the two frequencies can run apart to the maximum over thesynchronization window. In this respect, the synchronization can takeplace each time when the periods would theoretically have to coincide,in the example of 400 MHz and 410 MHz therefore every 100 ns, or onlyevery nth time, that is in multiples of 100 ns.

The time base unit preferably has a first PLL having a first divider ofthe master clock for the first frequency and a second PLL having asecond divider of the master clock for the second frequency, with inparticular the first divider and the second divider being selected suchthat a difference period which is as small as possible arises in therange of some hundreds or some tens of picoseconds or a few picoseconds.A numerical example is a master clock from 10 MHz and a divider pair40/41. Depending on the stability of the PLLs and of the presets of thedigital components used, larger dividers and thus shorter settingpossibilities can be provided. The two dividers should be co-prime withrespect to one another; they should preferably satisfy the relation nand n+1. A selection which is not co-prime does not produce anyimprovement, for instance at 5 and 10, or produces an improvement whichis not used ideally, for instance at 42 and 40.

The evaluation unit and/or the time base unit is preferably implementedon a digital logic component, in particular an FPGA (field programmablegate array), PLD (programmable logic device), ASIC (application specificintegrated circuit) or DSP (digital signal processor). Such digitalcomponents allow an evaluation adapted to the application and a simplegeneration of the required two frequencies, for instance when the FPGAalready has PLLs with settable dividers.

The time base unit preferably has a first counter and a second counterto count the complete periods of the first or second frequencies, withthe counters in particular having triggered shift registers and with thetime base unit being made to generate the time shift as a time intervalbetween the nth period of the first frequency and the mth period of thesecond frequency. A pair from specific periods of the two frequenciesdelivers time increments below a time pattern preset by the sampling. Ifthe frequencies satisfy the aforesaid n, n+1 relation, the sorting issimpler. It is sufficient if pairs are available to fill a samplingperiod since larger times can then be generated by addition of wholesampling periods. Alternatively, however, pairs can also be evaluatedbeyond a sampling period. The counters are reset accordingly with eachsynchronization or with each nth synchronization.

The time base unit is preferably made to extend the time shift byperiods of the first frequency, of the second frequency or of the masterclock. Time shifts which can be as much longer as desired can thus begenerated.

The evaluation unit is preferably made to trigger the transmission of alight signal at a transmission time in a respective measurement periodand to sample the received light signal and to accumulate a histogram ofsuch received light signals over a plurality of measurement periods todetermine the reception time from the histogram and the light transittime from it. Such a statistical approach also enables measurements whenthe individual measurement is very noisy, for example with unfavorablelighting scenarios, large distances or dark targets. The histogramapproach also increases the measurement precision under favorableconditions, however.

In an advantageous further development, a unit for the fine setting of atransmission time is provided which is made to shift the respectivetransmission time within the measurement periods by an offset, with theoffsets forming a distribution whose center of mass forms a desiredtransmission time and which can be selected with a time resolution whichis better than both the sampling period and the difference period, inparticular with a time resolution of fewer than ten picoseconds or evenless than one picosecond. It is not the discrete time pattern which isfurther refined here, but the time resolution is rather increaseddespite an existing time pattern beyond its resolution. In this respect,the time pattern refined by the time base unit forms a particularly goodstarting position for actual transmission times. The time position ofthe transmitted light signals is then admittedly not improved withrespect to the time pattern for the single shot, but very much so for agroup of single shots. The desired transmission time, that is ultimatelya phase of the group of single shots, is achieved via bin counts andthus actually via a center of mass set via statistical amplitudeinformation. The degrees of freedom for the center of mass position arein turn basically unlimited since it only depends on the number ofrepetitions, that is on the plurality of measurement periods. Timeprecision is thus compensated by response time, with this not playingany role for most applications since a sufficient number of repetitionsalready takes place in a very short time so that the monitored region orthe target can continue to be assumed to be quasi-statistical. Technicallimits for the setting of the actual transmission time of eachindividual light signal are thus overcome. The effective transmissiontime can be selected with practically any desired precision and thusenables very high measurement precision. The desired transmission timesare reliant neither on a time pattern of the digitization or a workcycle of a digital component nor on the smallest possible shift for thetransmission time. The precision reached in the picosecond range or evenbelow it is not achievable for the sampling itself or only with a verylarge effort and/or cost. This embodiment invention ultimately makes itpossible by a skillful programming of a digital component, that is by avery cost-favorable solution, to dispense with such complex hardware orto overcome the limits of such hardware.

It must be emphasized in this respect that transmission times are ineach case not to be understood absolutely, but relatively to thereception time. It is thus therefore absolutely possible to consider thesituation from a different perspective and to speak respectively ofshifted reception times or of a fine adjustment of the reception time.This will not be differentiated in language in the following and in theclaims. The interval between the transmission time and the receptiontime can in particular in each case be shifted in time as a wholewithout it affecting the measurement result. Such a common shift oftransmission time and reception time is consequently not meant bytransmission time delay; it can always optionally take placeadditionally. In a similar manner, terms such as offset or transmissiontime delay cover shifts on the time axis both in the positive and in thenegative direction.

The distribution of the offsets is preferably unimodal, in particular inaccordance with a triangular, parabolic or Gaussian function, with amemory being provided in which a table for the unit for the fine settingof a transmission time is stored which holds an associated offsetdistribution for a plurality of time increments, in particular onerespective offset distribution for time increments distributed evenlyover a sampling period and/or over the time interval between twodiscrete times. Such distributions have a particularly pronounced centerof mass and thus high time precision. In this respect, the distributionis formed from some sampling points which correspond to actualtransmission points and from associated counts, that is repetitions forparticularly this offset and thus ultimately amplitude information. Thefunctions given in this respect form an envelope over the bins. Thenumber of the sampling points should be selected in accordance with acompromise from a distribution which is as tight as possible and from asufficiently precise imaging of the center of mass and of the envelope,that is for example from 3 to 11 sampling points, or particularlypreferably from 5 to 7 sampling points. In this respect, thewell-defined center of mass is generally more important than thefaithful reproduction of the envelope so that discretization errors inthe setting of the distribution are preferably taken into account at thecost of the shape and not of the center of mass.

A Gaussian distribution is in particular advantageous since it not onlyhas a defined center of mass, but is rather also robust with respect tojitter. In contrast, jitter due to fluctuations in the ambient light ortolerances of the electronics is even desired. The discrete samplingpoints in the distribution are thus smeared into one another; theplurality of transmitted light pulses thus not only forms a discreteapproximation to a Gauss, but even an almost continuous Gauss. If it isassumed that the jitter corresponds to white noise, the Gauss willthereby possibly be a little distorted, but maintains its essentialproperties.

The table used is in precise terms a table of tables: A separate tableis stored for each time increment which can be set by distributions,namely a table with the counts which are required with respect to thesampling points and which thus gives the distribution. What was saidabove, that the well-defined center of mass in accordance with the timeincrement is more important than the faithful imaging of the envelopeapplies to every single one of these distributions because a shiftedcenter of mass would already introduce a measurement error induced bythe principle. The transmission time can be displaced by the timeincrement as desired with the help of the table. It is sufficient inthis respect if the table holds entries up to the next rougher period,that is up to the settable actual transmission times; however, it cangenerally also include more entries.

A level determination unit is provided in an advantageous furtherdevelopment which is made to utilize the area of the received signalrecorded in accumulated form on the histogram as a measure for thelevel, in particular by forming the sum of amounts over the bins afterthe noise level had previously been subtracted from each bin. The noiselevel can be determined as the mean value over all bins, for example.The sum of amounts of the received signal is not necessarily formed overthe total histogram, but also only over the time range in which thereceived signal is disposed. This is the better measure since otherwisethe noise-caused fluctuations are included in the level measurement.Conversely, the noise level is also not formed over all bins, but overbins outside the region of the received signal, preferably in the regionnot optically visible.

The evaluation unit is preferably made for a distance correction whichcompensates a remission-dependent shift on the basis of the levelmeasurement. The remission-dependent correction or black-and-whitecorrection required for this, that is the relationship which indicates acorrection value for the light transit time for each level, can betaught in advance and can be stored as a table or as a correctionfunction. The level measurement can also be evaluated to check the stateof the optical components, for example, adjustment, contamination or thetransmitted light speed.

The evaluation unit is furthermore preferably formed for a time encodingprocess in which the transmission times are acted on by an additionalencoding offset and this is subtracted again for the evaluation, inparticular by randomized or determined mixing of the distribution or byan additional center of mass shift. Such encoding processes have thepurpose of differentiating the transmitted light signal frominterference light, with interference light also being able to be a latereflection of a self-transmitted light pulse or of a sensor of the sameconstruction. Such interferers are smeared by direct jumping on the timeaxis, which can be compensated in the evaluation, or by “blurring” anddo not stand out, or at least do not stand out as much, from the noiselevel. Alternatively or additionally, the signal shape itself can beencoded, that is the shape of every individual signal, to recognize itsown light signal on the reception.

The evaluation unit is preferably made to check with reference to atransition condition of the received light signal whether the lightsignal is received at an observation time, with a regulator beingprovided which is made to adjust the transition time or the desiredtransmission time by means of an additional transmission delay time suchthat the transition condition is satisfied at the observation time. Inthis respect, the underlying idea is not considering a measurement as aunique process in which, for example, the measured value is determinedonce and is output without the sensor then continuing to remain active.Instead, the then currently available information is used constantly tokeep the measured result current. A precise and valid measured value isalways available because the regulation always adjusts the measurement.Errors due to noise or dynamics in the monitored zone are avoided. Theregulator works without thresholds and so fast. The procedure would evenbe superior for a single measurement without subsequent regulationbecause the regulation algorithm locates the measured value in a muchshorter time than, for example, nested intervals or a sequential shift.If the regulator enters transient oscillation in a few cycles, a precisemeasured distance value is available over the required transmissiondelay time from this time onward. An approximation is already determinedduring the transient oscillation and then delivers a more and moreprecise measured value by the regulation.

It is accordingly regulated to an observation time which is fixedrelative to the transmission time and whose selection is largely asdesired, but is preset independently of the measurement. The observationtime always remains the same although the invention does not generallyprohibit changing it. This observation time only has to be known for theregulation; it is not changed by the regulation and does not influencethe regulation, provided it is only selected reliably. For example, theobservation time is placed to the maximum measured distance, to the endof a measurement interval shortly before the transmission of the nextlight signal or fractions thereof. The observation time which is thusalways the same is the sum of the transmission delay time which is setby the regulator and which is the control parameter for the regulatorand the light transit time so that the latter can be determined simply.Constant time portions such as electrical signal propagation times arebest eliminated in advance by calibration.

In an advantageous further development, a regulation time intervalwithin which the regulator can check the transition condition and canadjust the transmission delay can correspond only to a partial range ofa measurement range of the sensor, and a change of position monitoringunit is provided to check periodically the time at which the receivedsignal is received and, if this time is outside the regulation timeinterval, to set a new regulation time interval for the regulator, withthe change of position monitoring unit having an agent, that is aprocess which is active constantly or in regular assigned time slots andwhich is independent of the regulator and with the agent having the aimof locating a valid regulation time interval and of setting it for theregulator in which the light signal of the target object is actuallyreceived.

The regulator thus always works in an environment of the measured valuebeing sought, that is converges fast and does not remain incorrectly ona noise signal or on a target which has become no longer present in themeantime. The location of the received signal is only possible andnecessary in the sampling pattern in this connection, not a precisemeasurement, so that the regulator is given a sensible working range.For example, the regulation time interval can be selected such that itcontains a monotonous portion of the first falling flank of the receivedsignal to be able to regulate a jump to the first zero crossing withoutrisk. Presetting a regulation time interval therefore means the roughsetting of the transmission delay time. The observation time is notchanged in this respect; at least, it is not necessary if it wasinitially selected with a sufficient interval. This procedure allows avery fast approximation to a new measured value.

A (software) agent decouples the actual regulation and the location ofthe regulation time interval; it is therefore more robust and easier tohandle. The agent does not only have the aim of initially finding acorrect regulation time interval, but rather of always checking thisregulation time interval and of correcting it, where necessary, that isto carry out a constant adjustment of the measured value as the result.The agent thus reacts in a higher ranking manner to noise or dynamics inthe monitored zone and reacts by a setting of the regulator to asensible regulation time interval in which the received signal beingsought or the signature being sought is really to be found. Theindependence of the process can actually be implemented in its ownhardware path or, in software language, in the sense of a separatethread or task. It can, however, also only be understood conceptionally,whereas the real implementation, for example, implements the agent as aperiodically called up part of the regulator.

A filter element is preferably provided in the reception path betweenthe light receiver and the evaluation unit to convert the unipolarreceived signal into a bipolar signal, with the transition condition inparticular including a zero crossing from the first maximum to the firstminimum of the bipolar signal. A (post) oscillation is also covered bybipolar signal. The transition condition corresponds on thetime/distance axis to the desired value of the regulation or to thevalue of the distance to be determined. The filter can be part of thedigital component of the evaluation unit; however, it is preferably ananalog component since otherwise too many signal portions are alreadylost beforehand and the precision is impaired. The filter can, forexample, be a differentiator or a band pass. It is conceivable to definethe transition condition via a different and also more complexcharacteristic, that is a later zero crossing or a point of inflection.The extremes themselves could be used for this purpose between which thezero crossing is disposed, but whose characteristic is level-dependentand therefore less robust or more characteristics or zero crossingscould be used to increase the precision further.

In this respect, the change of position monitoring unit or theevaluation unit is preferably made to determine the noise level as thereference point in advance. For this purpose, averaging can be carriedout over all the bins or a selection of the bins in the histogram.

The change of position monitoring unit is preferably made to recognizethe received signal with reference to a signature, in particular to analternating change from maxima to minima and vice versa which each forma falling envelope, in particular a logarithmic envelope. A signaturedetects the substantial features of a function curve and can thus beevaluated and can be recognized faster despite fluctuations, in contrastto a comparison with the total function curve. This signature can besimple or complex, depending on whether the evaluation time or theprecision is the focus. It should be robust against noise, fast to beevaluated and as inimitable as possible. How many of the alternatingchanges have to be present for this purpose and how precisely theamplitudes of the associated envelope have to be met can accordingly beoptimized for the application. The signature can be located severaltimes over the total monitored zone, for example by multiplereflections. The respectively most pronounced signature should thereforedetermine the fixing of the regulation time interval, which is often theone which starts with the strongest maximum which is found in themonitored zone. The signature should be selected and characterized suchthat the regulator can find the transition condition.

The change of position monitoring unit is preferably made to store ahistory of which regulation time interval it would in each case havepreset for the regulator at the periodical check to preset thatregulation time interval for the regulator which is that of the receivedsignal in accordance with a statistical evaluation of this history.Short or single events are thus initially not taken into account so thata precipitate jump is avoided. Only when a better regulation timeinterval is found more sustainably is the regulator also offset. In thisrespect, a specific inertia for the then current regulation timeinterval is preferred which can be reflected in a higher statisticalweighting in the history. The then current regulation time intervalshould be preferred, in particular when the statistical evaluationcannot decide or can only make a close decision between two or moreregulation time intervals, for so long until a clear decision can bemade.

The method in accordance with the invention can be designed in a similarmanner by further features and shows similar advantages. Such furtherfeatures are described in an exemplary, but not exclusive manner in thedependent claims subordinate to the independent claims.

The invention will also be explained in the following with respect tofurther advantages and features with reference to the enclosed drawingand to embodiments. The Figures of the drawing show in:

FIG. 1 a very simplified schematic block diagram of an optoelectronicdistance measurement sensor in accordance with the invention;

FIG. 2 a block diagram of the sensor in accordance with FIG. 1 with arepresentation of further elements;

FIG. 3 a schematic representation of the signals in different processingstages for the explanation of the evaluation process;

FIG. 4 an overview representation of the individual processing blocksfor the digital signal evaluation;

FIG. 5 a block diagram for the generation of a high-resolution timebase;

FIG. 6 schematic signal curves for the explanation of the generation ofthe time base;

FIG. 7 a schematic representation of the transmission patterns toincrease the resolution;

FIG. 8 a representation in accordance with FIG. 7 for the explanation ofthe generation of high-resolution time increments;

FIG. 9 a representation of the time intervals and of the observationtime to which the reception of the light signals is regulated;

FIG. 10 a representation analog to FIG. 3 for the further explanation ofthe regulation in accordance with FIG. 9;

FIG. 11 a schematic representation for the explanation of a higherranking monitoring agent to the correct regulation time interval inwhich the observation time is changed;

FIG. 12 a representation in accordance with FIG. 11 with examples forinterference signals to which the monitoring agent does not change;

FIG. 13 a representation of a received signal for the explanation of alevel measurement; and

FIG. 14 a schematic representation of a transmission pattern encodingfor the measurement zone extension and/or for the safe association ofthe transmitted signal to the received signal.

FIG. 1 shows an optoelectronic distance measurement device or sensor 10,which is shown very simplified and which transmits via a lighttransmitter 12 a light pulse to a reflector or to a reflecting targetobject 14. The light beam reflected or remitted there returns to a lightreceiver 16 which surrounds the light transmitter 12. Because the lightbeam expands on its path, the light transmitter 12 only covers a smalland insignificant portion of the reflected light. Alternatively, otherknown solutions can naturally also be used such as autocollimation witha beam splitter and a common optical system, for instance, or pupildivision, where two separate optical systems are provided and the lighttransmitter and the light receiver are arranged next to one another.

The light transmitter 12 and the light receiver 16 are controlled andevaluated by a control 18. The control 18 causes the light transmitter12 to transmit individual light pulses at a known time. It will beexplained in detail further below how the required transmission timedelay is achieved. The control 18 determines the reception time of thelight pulse in the light receiver 16 in a manner likewise still to beexplained. The light transit time which in turn corresponds via thespeed of light to the distance of the target object 14 is calculatedfrom the reception time with the known transmission time.

At least two modes are possible for the sensor 10. In one mode, thelight transit time and thus the distance is measured. In another mode, aspecific distance is taught, for example with respect to a fixedcooperative target, and monitoring is carried out whether its distancechanges.

The sensor 10 can be an optoelectronic sensor or a distance measuringdevice. In addition to an actual distance measurement, in which anabsolute value is determined for a distance to an object 14, themonitoring of a taught distance, for example from a fixed cooperativetarget 14, for changes of the taught distance is also conceivable. Afurther embodiment is a reflection light barrier, that is a lightbarrier having a light transmitter and a reflector arranged opposite,with an interruption of the beam reflected there being detected.Monitoring can be done by the measurement of the distance or of thechange of the distance of this reflector whether the reflector is stillat the expected position. All the known sensors can output or display adistance value or can also work as a switch in that a switch event istriggered on detection of an object at a specific distance or on adeviation from an expected distance. A plurality of sensors 10 can becombined, for instance to form a distance-measuring ordistance-monitoring light grid. Mobile systems are also conceivable inwhich the sensor 10 is mounted movably or scanning systems in which thetransmitted light pulse sweeps over a monitored line or a monitored areawith a deflection unit, with the deflection unit being able to be arotating mirror or a polygonal mirror wheel.

Further details of the sensor 10 are shown in FIG. 2. Here and in thefollowing, the same reference numerals designate the same features. Alaser diode 12 is shown as the light transmitter by way of example here.Any desired laser light sources 14 can be used, for example edgeemitters or VCELs (vertical cavity surface emitting lasers), andgenerally other light sources such as LEDs are also suitable providedthey can generate signals sufficiently sharp in time. The receiver isaccordingly shown as a photodiode 16, with the use of a PDS (positionsensitive diode) or of an array or of a matrix of light receivingelements being conceivable such as a CMOS chip, that is generally anyreceiver which can convert a light signal into an electric signal.

The control is implemented in the described embodiment in accordancewith the invention on an FPGA (field programmable gate array) 18.

Alternative digital components were already named non-exclusively in theintroduction. The control 18 has a transmission time setting device 20and an actual evaluation unit 22. The terminals of the FPGA 18 are madedifferently to be able to transmit the signals more free of interferencesignals. The target object 14 is usually further away in the scale ofFIG. 2, as is indicated by dashed lines 24.

The sensor 10 has a transmission path to which, in addition to theactual light transmitter 16, a laser driver 26 and the delay device 20belongs, and a reception path to which the photodiode 12 which suppliesthe digitized received signal to the evaluation unit 22 via an analogpreprocessor 28.

The analog preprocessor 28 forms a multi-stage processing path. Thisstarts with an amplifier 30, for instance a transimpedance amplifierwhich accepts and amplifies the signal of the photodiode. A downstreamfilter 32, which can be a band pass filter or a differentiator, forexample, converts the unipolar light signal into a bipolar signal. Alimiting amplifier 34 is provided as the next preprocessing stage whichamplifies the amplitude so much and subsequently cuts it so that thelight pulse signal is driven to a rectangular pulse driven intosaturation. This signal is supplied as the last preprocessing stage toan A/D converter 36, in particular to a binarizer, which does notconvert the amplitude into a digital numeric value, but only into abinary value. The A/D converter 36 is preferably not a separatecomponent, but is rather realized via the inputs of the FPGA 18 withsimple analog R networks or RC networks connected upstream.

The signal and evaluation path in the sensor 10 through the componentsjust described will now be described with the help of FIG. 3. In thisrespect, a statistical evaluation of a plurality of individualmeasurements is preferably provided because the signals of theindividual measurement have much too much noise to be able to determinereliable reception times.

The light transmitter 16 in each case generates a light pulse in eachmeasurement period 100 which enables the determination of a precisetime. As explained further below, the control 18 distinguishes aregulation time interval 101 which only includes a part of themeasurement period and corresponds, for example, to a meter ofmeasurement path. A rectangular pulse is suitable as the light signal,but other pulses, such as Gaussian pulses, multimodal signals, for theencoded association of each signal, for example, and also stages areconceivable. All these signal forms will only be called a light pulse inthe following.

The light pulse is reflected or remitted at the target object 14 in themonitored zone of the sensor 10 and is then converted into an electricalsignal in the light receiver 12. The electrical signal is subsequentlyamplified in the amplifier 30. The amplified electrical signal 102 whicharises is shown in idealized form; under realistic conditions, thereceived light pulse 102 would not show a precise rectangle, but wouldonly show transients at the flanks and noise overall.

The amplified electrically received light pulse is a unipolar signal dueto the nature of the light. It is converted to a bipolar signal 104 inthe filter 32. This can be realized with a band pass filter, but thegenerated signal curve 104 corresponds at least approximately to theextended derivation of the amplified signal 102. In FIG. 2, grayrectangles are shown, beside the bipolar signal 104, which are intendedto symbolize the noise level. The noise level can surpass the amplitudeof the amplified signal 102 in practice. Furthermore, only a sineoscillation of the bipolar signal 104 is shown. Post-oscillations, thatis further sinus periods with increasingly damped amplitude, are omittedfor a simplified representation. A pure sine is naturally also notalways to be expected, but a curve with a maximum and a minimum.

The bipolar signal 104 is amplified so much and cut-off in the limitingamplifier 34 such that the actual signal becomes a rectangle flank 106and the noise level shown by the gray rectangles is extended over thetotal dynamic range in its amplitude.

The rectangle flank 106 is sampled with a sampling rate of, for example,2.5 ns in the binarizer 36. This sampling rate is symbolized by arrows108 in FIG. 3. The bit sequence which arises, 1 bit per 2.5 ns with thenumerical values given, is used in the evaluation unit 22 to form ahistogram 110. An accumulator is provided for each bin for this purposeand is only counted up with an associated bit value “1”. The samplingis, contrary to what is shown, not necessarily limited to the regulationtime interval 101.

With ideal signals without noise, only that bin would be filled in thishistogram 110 which is disposed above the right hand flank 106. Thenoise level raised by the limiting amplifier 34, however, also fills theremaining bins, and indeed approximately in every second measurementperiod 100 due to the randomness of the noise in the expected value.

If the process just described is iterated and if the histogram 108 isformed over k measurement periods 100, the bins are filled approximatelywith the value k/2 by the noise, with statistical fluctuations beingadded. This value k/2 corresponds to the signal value zero due to thebinarization. The maximum formed by the positive part of the bipolarsignal 104 rises upwardly from this and the corresponding minimumdownwardly. Together with the post-oscillations, not shown, thehistogram shows a characteristic curve in the time interval of thereceived signal whose signature is used by the evaluation unit 22 todetermine the reception time. The statistical evaluation of a pluralityof individual measurements also allows this when the individualmeasurement does not permit any reliable distance determination in ameasurement period 100 due to noise portions which are too high.

Due to the limited sampling rate, which is given by way of example at2.5 ns, it is not sufficient to search directly for the received signalin the histogram 110 since the time resolution would be too low. FIG. 4shows an overview representation of the procedure in accordance with theinvention to improve the time resolution far beyond the precision of atime pattern preset, for instance, by an FPGA or an A/D converter. Inthis respect, a plurality of mutually engaging steps are shown in theoverview of FIG. 4. The best total performance is achieved in thiscombination. It is, however, not absolutely necessary to implement allof the steps simultaneously. A partial selection also already increasesthe measurement precision with respect to conventional systems. Theindividual steps in accordance with the overview in FIG. 4 willsubsequently be explained in more detail with further Figures.

The transmission time setting device 20 has a time base unit 38 whichprovides a high resolution time base using a process based on twofrequencies. The time base can be utilized to delay the transmission oflight pulses with much more precision than with multiples of 2.5 ns, forexample with multiples of 60.975 ps.

The transmission time setting device 20 furthermore has a unit 40 forthe fine setting of a transmission time in which a transmission pattern,for example a Gaussian transmission pattern, is formed by means of aplurality of individual measurements to refine, theoretically asdesired, an effectively acting transmission time delay by the center ofmass of the associated reception pattern with respect to the possiblephysical transmission times. The time base unit 38 therefore directlychanges the resolution which is further refined indirectly by the unit40 for the fine setting of a transmission time via a statistical centerof mass shift.

The light pulses conducted over the measurement path on such a highlyresolved time pattern are received and are digitized in the A/Dconverter. Subsequently, the histogram evaluation explained with respectto FIG. 3 takes place in a histogram unit 42.

The actual distance determination takes place in a regulator/agent 44and is not based on a direct sampling, but rather on a technicalregulation adjustment principle to use the generated time resolutioneffectively. The regulation parameters have to be dimensioned in thisrespect, on the one hand, such that required stability criteria aresatisfied and the sensor 10 remains robust with respect to interferenceinfluences, for example by further reflections or EMC. On the otherhand, however, this has the consequence of too low an agility of aclassical regulator which could no longer react threshold-free to a realtarget change. The invention therefore provides monitoring the regulatorconstantly by means of an agent in the background. The agent regularlyevaluates the total working region of the sensor 10 and controls theregulator to the correct regulation time interval 101, that is the timerange of the target position, on a change of target.

The histograms 110 for a high resolution level measurement can beevaluated in a level determination unit 46. Customarily used additionalanalog elements can thus be dispensed with. Furthermore, the leveldetermination is very precise, especially in combination with theregulation principle. The level can be output, but also be used for acorrection of the distance measurement.

The transmitted pulses can be output in encoded form on the time axis inan encoding unit 48 to enable an unambiguous association of transmittedpulse to received pulse. They are then decoded in a decoding unit 46which was combined with the level determination unit in FIG. 4 forsimplification. It can, for example, be achieved using a transmissionpattern encoding to suppress received pulses from the background, thatis those which are received outside the measurement zone after the endof the actually associated measurement period 100. Light pulses fromsystems of the same construction represent another possibility ofconfusion which is prevented by encoding. In this respect, the Gaussiantransmission pattern is not transmitted and received in a natural order,but rather in a randomized order. The decoding unit 46 knows therandomization key and can thus perform reverse encoding. A plurality ofcode signatures can thus be underway simultaneously on the light signalpath because the different path sections are characterized by theencoding and are thus unambiguous.

The process will now be explained with reference to FIGS. 5 and 6 withwhich the time base unit 30 provides time increments independently ofthe sampling rate of 2.5 ns, for example in a time pattern of 60.975 ps.

A split clock is generated from a master clock 50 of 10 MHz as amultiple of the master clock 50 of f1=400 MHz or f2=410 MHz in a firstPLL 52 (phase-locked loop) and a second PLL 54. The time base unit 38receives the two frequencies of the PLLs 52, 54 and the master clock 50itself for the synchronization. The frequencies are connected in thetime base unit 38 such that their phase deviation can be used for thereproducible generation of time increments. The frequency of 400 MHz ofthe first PLL 52 simultaneously serves as a sampling rate for the A/Dconverter 36.

As can be seen in FIG. 6, the periods of the two different frequencies400 MHz and 410 MHz increasingly run apart and meet again after a periodof the master clock 50 of 100 ns. At this time, synchronization in eachcase takes place to the theoretically simultaneously rising or fallingflank so that any running apart of the PLLs 52, 54 and of the masterclock 50 is compensated. FIG. 6 is simplified and only shows 10 or 11periods instead of the actually required 40 or 41 periods.

The PLLs 52, 54 are preferably provided by the FPGA 18. The twofrequencies can, however, also be generated differently than by means ofPLLs. A master frequency deviating from 10 MHz and different frequenciesthan the exemplary frequencies f1=400 MHz and f2=410 MHz are naturallyalso covered by the invention, with the choice having to find a balancebetween the stability of the derived frequency generated and adifference period which is as short as possible. Time patterns in therange of picoseconds and below can be achieved at least in principle bythis choice.

The periods of the derived frequencies f1 and f2 are counted through inshift registers triggered by these frequencies so that the time baseunit 38 as shown in FIG. 6 knows which period a flank belongs to. Anincreasing phase difference forms between the respective ith period off1 and f2 and is just so large after a full period of the master clock50 that the 41st period of f2 comes to lie simultaneously with the 40thperiod of f1. These differences are available in the form of timeincrements or time budges as multiples of the difference periodΔT=1/f1−1/f2=60.975 ps. In this respect, reference is again made to thenumbers 10 and 11 of FIG. 6 which differ for the simplifiedrepresentation.

The time base unit 38 now selects a respective pair from the nth periodof the frequency f2 and the mth period of the frequency f1 to generateany desired multiples of the difference period. Each pair has a fixedposition relative to the master clock 50. For example, n=2 and m=6corresponds to a time interval of 4/f2+6ΔT, where 1/f2=41ΔT. Fullperiods of the master clock will be added in this respect to fill themeasurement period 100 of 1 μs, for example by a higher ranking controlunit which masks the timing and is fixed to the master clock. In thisrespect, the counters are reset after 100 ns with each synchronizationso that the numbering of the pairs starts again. Provided that theperiods of f1 and f2 are counted beyond the synchronization time after100 ns, the pairs can alternatively also directly fix time intervalslonger than 100 ns. To be able to decouple the pairs in a definedmanner, the two derived frequencies f1 and f2 should have a rigidcoupling to the master clock as is given by PLLs.

Due to the two derived frequencies f1 and f2, a time base is thusavailable which is substantially finer than the sampling pattern. Eitherthe actual transmission time can thus be delayed with respect to areference time by multiples of the difference period or the one elementof the pair defines the transmission time and the other the time for thestart of the statistical recording of the received pattern in thehistogram unit 42. There is thus a time offset between the transmissiontime and the reception time which is independent of the sampling patternwith the slow 2.5 ns. The time base unit 38 can work completely withinthe FPGA 18 and can therefore be implemented simply and is less prone tointerference.

The time increment available by the time base unit 38 is furthermorediscrete and is determined by the selection of the frequencies. Theprecision of an individual measurement within a measurement period 100is therefore initially limited by the difference period of the selectedfrequencies.

FIGS. 7 and 8 illustrate a method for the time resolution increase for aplurality of individual measurements by means of the unit 40 for thefine setting of a transmission time. In this respect, the transmissiontime is varied with reference to a distribution in the repetitions infurther measurement periods 100. In accordance with an envelope 56, theassociated occurrences are preset at the discrete sampling points 58which are fixed by the physically possible discrete transmission times.The center of mass of this distribution determines the effectivelyactive transmission time which is decisive for the total statisticalevaluation for the histogram 110 after k measurement periods 100.

This center of mass is now, however, not bound to the discrete physicaltransmission times or sampling points 58 themselves. By selection of adifferent distribution 60, that is of different occurrences 62 at thesame discrete sampling points, the effectively active transmission timecan be selected with a precision which can be increased theoretically asdesired also between the discrete sampling points 58. In FIG. 7, thesampling points 58 of the one distribution shown striped are shownslightly offset with respect to the sampling points 62 of the otherdistribution 60 shown dotted. This serves only for illustration sincethe sampling points are respectively bound to the same discretephysically possible transmission times. The sampling point grid can beunderstood as possible offsets with respect to a reference time and thusthe occurrences can be understood as an offset distribution.

FIG. 8 illustrates how fine time increments can be defined in thismanner. The starting position for a time increment Δt₀=0 is shown in theleft hand third of FIG. 8 at which the individual measurements shown asblocks 64 form a distribution whose center of mass time t_(CoM) justcoincides with the reference time t_(ref). In precise terms, it is notnecessary already to work with a distribution at all here since thecenter of mass time t_(CoM) would also be achievable directly via thediscrete time pattern.

A distribution is now chosen for the next time increment whose center ofmass is shifted a little as is shown in the middle third and in theright hand third of FIG. 8. For this purpose, some individualmeasurements are carried out with different offsets. For example, threeindividual measurements are shifted to the right in each case asindicated with arrows 66. It is naturally conceivable to select adifferent number than three, with only one shifted individualmeasurement fixing the lowest possible time increment. If the number isvaried from step to step, the arising time grid is irregular.

Analogously, a plurality of distributions can be set forth at which therespective center of mass time t_(CoM) is increasingly shifted by Δt₁,Δt₂, . . . with respect to the reference tine t_(ref). The discrete timepattern of the sampling points is thus refined by the distributions andassociated center of mass times t_(CoM) with a table of suchdistributions which fills the interval between two sampling points. Theunit 40 for the fine setting of a transmission time can make use of thistable to output a transmission pattern with a desired time increment andthus to achieve a desired or effectively active transmission timeindependently of the discrete sampling points.

The distribution which is fixed by the envelope 56, 60 should have moremass in the proximity of the center of mass. Unimodal distributions, forexample triangles, parabolas or a Gauss curve are therefore preferredwhich also each have a small standard deviation so that the measurementbase does not become too wide. A few sampling points are alreadysufficient for this purpose. On the other hand, the flank should notdrop too steeply, so that a Gaussian profile is preferred.

A certain noise in the system even benefits the process in this respectsince then the sampling points are quasi smeared into one another andform a smoother approximation to the envelope 56, 60. A completelynoise-free system would obtain artifacts of the discrete sampling pointsin the reception pattern. Since generally interference approximatelyresults in Gaussian noise, a Gaussian distribution is again preferredfor the envelope 56, 60.

The achievable increase in resolution ultimately depends only on thenumber of individual measurements k which flow into the formation of thehistogram 110. Each additional measurement creates further possibilitiesto define additional time increments, as illustrated in FIG. 8. Withsome hundred repetitions, for example, the response time up to theavailability of a distance measurement value is still at some hundredmeasurement periods 100, that is at some hundred μs with the numericalvalues of FIG. 3. An increase in resolution by approximately two ordersof magnitude can thus already be achieved. If the discrete time patternof the sampling points is fixed by the time base unit 38 at, forexample, 60.975 ps, a sub-picosecond resolution is thus made possible.

Despite the two previously introduced possibilities for the refining ofthe discrete time pattern, the resolution of the histogram 110 is stilllimited per se by the sampling rate of the A/D converter 36. In ordernow to fully profit from the increase in resolution, in accordance withthe invention, no attempt is made to determine the reception time withhigh precision, but it is rather fixed in advance as an observation timeand subsequently a transmission time delay is adjusted for so long untilthe reception time coincides with this observation time.

FIGS. 9 and 10 illustrate this regulation. The observation timet_(Control) is selected in advance at a point of the sampling gridsomewhere within the measurement period 100 such that it is behind themaximum light transit time to be measured, for example at the center ofthe measurement period 100 at 0.5 μs or approximately 75 meters. Thelight pulse is delayed by a transmission time delay with the help of thetime base unit 38 and/or the unit 40 for the fine setting of atransmission time with respect to a common time reference t_(Start)before the transmitted pulse is actually transmitted at a time t_(Send).After the light transit time which is the actual measurement parameter,the light pulse is received again at a time t_(Receive). It is theobject of the regulation to correct the transmission time delay in afeedback loop such that t_(Receive) always coincides with t_(Control) asis shown by regrouping of the hatched blocks 67 a, b.

The light transit time can then be calculated by simple subtraction. Thetime interval t_(Control)−t_(Start) is a known constant selected inadvance which differs in the steady state condition precisely by thetransmission time delay from the light transit time. Further constantportions, for instance signal transit times in the electronics, can beeliminated by calibration or taken into account in the calculation. Atemperature compensation is also possibly required for these portions.

The regulator must be able to recognize with high precision for thefeedback whether the reception time t_(Receive) coincides with theobservation time t_(Control). FIG. 10, which coincides in large partswith FIG. 3, illustrates this. The observation time is marked by a boldarrow. The zero crossing from the first maximum to the first minimum ofthe received signal recorded as the histogram 110 is monitored as thetransition condition which fixes the reception time. Othercharacteristics can naturally also be evaluated, but the first zerocrossing is the most pronounced and is largely independent of the levelin contrast to the extremes themselves.

The hatched rectangle 70 of FIG. 10 indicates the deviation inaccordance with the hatched rectangles 67 a, 67 b of FIG. 9 from theideal transition position. This is therefore a measure for the controldeviation and the basis for the calculation of the required adaptationof the transmission time delay. If the signal transition t_(Receive) isdisposed in the observation time t_(Control), this control deviation canbe corrected to zero at least in the ideal system by adjusting thetransmission time delay.

The regulation is implemented digitally in the FPGA and thus has accessto the histogram 110. The regulation process per se can include anyknown variant, for example Kalman-based regulation, or the regulator isa PI regulator or a PID regulator.

The regulator preferably does not work over the total measurement period100, but rather only within a regulation time interval 101, and it isfavorable for the avoidance of erroneous regulations if it is smallenough not to include a plurality of potential targets 14. If the signaltransition t_(Receive) is not within this regulation time interval 101,the regulator cannot determine the regulation deviation 70. A higherranking agent is therefore provided which in each case searches thehistogram 110 for potential targets 14 over the total measurement zone.The agent is either a separate process or is at least conceptionallyseparate from the regulation which then periodically calls it up and isthus higher ranking than the regulation. Even if the regulation timeinterval 101 is selected as wide as the measurement period 100, theregulator itself cannot easily recognize a target change since there isthe risk that it would converge at local extremes and would not exitthem independently.

The agent preferably does not recognize the received signal withreference to a complete pattern comparison since this would be toonoise-sensitive. Instead, it searches for a signature which can begiven, for example, by the alternating regular transition from thepositive to negative maximum amplitude and vice versa. The signature cancorrespond to higher demands the more such changes of sign are monitoredand it is conceivable to demand further criteria such as the observationof the logarithmic fall of the absolute values. These exemplarysignatures apply to a simple oscillation with a positive and a negativesignal portion which arise from a simple transmitted pulse. More complextransmission signals are conceivable to harden the system with respectto external interference or systems of the same construction and thesignature is then also to be selected in correspondingly adapted form.

FIG. 11 shows an example for a target change. The regulation timeinterval 101 is initially selected around a signal 72 and the regulatorhas corrected the observation time to its first zero crossing. Thehigher ranking agent has, however, located a more pronounced signal 74in the meantime. To carry out the target change, the agent calculates atime difference 76 and sets the regulator to the new signal 74 in thatthe regulation time interval 101 is shifted, that is the transmissiontime delay is adapted by the time difference 76. In this respect, theagent in no way has to calculated the precise time difference 76 asshown in FIG. 11, but it is rather sufficient if the regulation timeinterval 101 is selected roughly around the signal 74 so that theregulator can adjust to the exact new reception time.

A plurality of conditions have to be satisfied for such a change ofposition or target. A check is initially made where signals with thedemanded signature are then currently located. In this respect, evensimple threshold evaluations can prefilter. The noise level, which is atk/2 in the ideal case, is taken into account by mean value formationover the histogram 110 or partial regions thereof. The maximumamplitudes of such located potential targets are subsequently compared.If a potential target with a larger amplitude is located outside thethen current regulation time interval, this potential target representsthe actual then current target 14 from the point of view of the agent.However, so that singular events or misinterpretations of the agent donot result in unnecessary jumps, the agent records a history of thepotential targets with a defined decay time, for example in a queue.Only when a new target accumulates statistically significantly in thishistory does the agent actually carry out a target change, for exampleif a specific target were selected in 5 of 8 cases within the history.In this manner, the system can change to a new position withoutthreshold, thus allows measurements up to very low signal levels and isnevertheless robust with respect to interference.

FIG. 12 shows two cases of potential targets which do not satisfy atleast one of the named criteria and which therefore also do not triggerany target change. In addition to the signal 72 of the then currenttarget 14, a respective further potential target is given by signals 78and 80. The signal 78 also satisfies the signature, but has a smalleramplitude and is therefore not selected. In this respect, the amplitudecan still be distance-corrected. The signal 80 already does not satisfythe signature and is therefore directly recognized as interference.

The received signal 104 or the histogram 110 also contains levelinformation on the area below the signal in addition to the distanceinformation over the time position. On a linear observation, the levelis proportional to the quantity area below the oscillation. A levelmeasurement value is therefore available in a simple manner via afurther evaluation. FIG. 13 shows the course of a received signal 82together with the post-oscillations which are omitted in the otherFigures for simplification and with which the received signal 82logarithmically decays. As in numerous other cases, a bold arrow withinthe sampling pattern 108 indicates the observation time to which thefirst zero crossing of the received signal 82 is regulated. The summedsignal amplitudes 84 of the received signal at sampling points 108 are ameasure for the level.

Since the position of the received signal 82 is corrected such that thezero crossing is above a sampling point and the light pulse furthermorehas precisely a length of 5 ns, that is a multiple of the sampling rate,the further zero crossings are also just above sampling points. Thisfixing of the histogram 108 has the result that good level informationcan be derived despite the low sampling rate. The zero crossingsthemselves then namely do not contribute anything and, since theextremes are central in each case and thus also on a sampling point,only particularly significant amplitude information flows into the levelmeasurement.

If the received pulse 102 is shaped in the analog preprocessing 28 suchthat it shows a weakly resonant behavior and if furthermore a limitingamplifier 34 is connected downstream of the filter 32, the dynamic rangeof a level measurement considerably increases.

The level measurement is not only a possible output parameter, but thelevel information can rather also be utilized for the correction of alevel-dependent distance deviation. This effect, known under the nameblack-and-white shift, has the result that the determined reception timeshows a dependency on the intensity. If this dependency is taught at thestart or if it is taken into account in a correction calculation, thedetermined distance can be compensated and can be made independent ofthe level over a wide intensity range.

The level information can furthermore also be used for the adjustment ofthe optical components of the system. A contamination or maladjustmentis thus recognized, for example, or the power of the light transmitter12 can be adapted.

It is conceivable that the sensor 10 accumulates interference signals inthe histogram 110 which then result in incorrect measurements. For thispurpose, particularly received signals with respect to separatetransmitted pulses can be considered which are reflected beyond themeasurement zone or also systems of the same construction whose lightpulses are received. It is therefore desired to be able to associatereceived signals with a specific self-transmitted transmission pattern.The encoder 46 and the decoder 48 serve for this purpose which generateadditional compensatable shifts on the time axis and resolve them again.Such time shifts also have the effect that interferers constant in timeare smeared by the averaging because they lose the fixed time referencedue to the time encoding and are each recorded in a different bin.

In particular two kinds of time encoding can be considered which areillustrated in FIG. 14. The center of mass position Δt₁ . . . Δt_(n) isshifted in each measurement period 100, on the one hand. Only the actualreceived measurement signal follows this random and fast shift of thecenter of mass position so that interferers can be distinguished oraveraged out directly.

For a measurement zone extension or for systematic interference, forinstance due to multiple reflections or a system of the sameconstruction, the center of mass shift is not necessarily sufficient.For this reason, alternatively or accumulatively, the order can bevaried with which the Gaussian profile is generated in the unit 40 forthe fine setting of a transmission time. This order is of no interestfor the generation of the histogram 110 since only accumulation takesplace. This degree of freedom is used to select a different order withevery code 1 . . . n as is shown by way of example in FIG. 14 with thenumbered transmitted pulses 86. It is thereby in particular possible toexpand the measurement zone to multiples of the measurement period 100since is it clear via the encoding which partial zone a receivedtransmission pattern belongs to.

The time variations can be randomized or deterministic. Randomizationhas the advantage of providing differences to systems of the sameconstruction. The decoder 46 naturally also has to receive the offsetinformation with randomized shifts in order to be able to compensate it.

The individual elements of the overview FIG. 4 are thus explained indetail. Although the sensor 10 was described in its totality in thismanner, individual feature groups can also be utilized sensiblyindependently of one another. For example, the Gaussian transmissionpattern thus further refines the actual transmission times generated bythe two frequencies. Both steps, however, also already achieve anincrease in resolution per se. Accordingly, these and other featuregroups can also be differently combined, in particular over severalFigures, than described in the specific embodiments.

1. An optoelectronic sensor (10) for the measurement of distances ordistance changes in accordance with the light transit time principlehaving a light transmitter (12) for the transmission of a light signaland having a light receiver (16) for the reception of the remitted orreflected light signal, wherein an evaluation unit (18) is providedwhich is made to fix the transmission time for the light signal relativeto a reference time and to digitize the received light signal on asampling pattern (108) having a sampling period to determine thereception time of the light signal, and wherein a time base unit (38) isprovided by means of which the transmission time can be shifted relativeto the sampling pattern (108) with a time precision below the samplingperiod, wherein the time base unit (38) is made to derive the time shiftfrom a first time signal having a first frequency (f1) and a second timesignal having a second frequency (f2) different from the first frequency(f1) and to provide any desired time shifts with a time resolution givenby the difference period belonging to the first and second frequencies(f1, f2).
 2. A sensor (10) in accordance with claim 1, wherein the timebase unit (38) is made to derive the first frequency (f1) and the secondfrequency (f2) from a master clock (50) which also determines thereference time and to synchronize the first frequency (f1) and thesecond frequency (f1) to the master clock (50) regularly.
 3. A sensor(10) in accordance with claim 2, wherein the time base unit (38) has afirst PLL (52) having a first divider of the master clock (50) for thefirst frequency (f1) and a second PLL (54) having a second divider ofthe master clock (50) for the second frequency (f2), and with inparticular the first divider and the second divider being selected suchthat a difference period which is as small as possible arises in therange of some hundreds or some tens of picoseconds or a few picoseconds.4. A sensor (10) in accordance with claim 1, wherein the evaluation unit(18) and/or the time base unit (38) is implemented on a digital logiccomponent, in particular on an FPGA, PLD, DSP or ASIC.
 5. A sensor (10)in accordance with claim 1, wherein the time base unit (38) has a firstcounter and a second counter to count the complete periods of the firstor second frequencies (f1, f2), with the counters in particular havingtriggered shift registers and with the time base unit (38) being made togenerate the time shift as a time interval between the nth period of thefirst frequency (f1) and the mth period of the second frequency (f2). 6.A sensor (10) in accordance with claim 1, wherein the time base unit(38) is made to extend the time shift by periods of the first frequency(f1), of the second frequency (f2) or of the master clock (50).
 7. Asensor (10) in accordance with claim 1, wherein the evaluation unit (18)is made to trigger the transmission of a light signal at a transmissiontime in a respective measurement period (100) and to sample the receivedlight signal and to accumulate a histogram (110) of such received lightsignals over a plurality of measurement periods (100) to determine thereception time from the histogram and the light transit time from it. 8.A sensor (10) in accordance with claim 1, wherein a unit (40) for thefine setting of a transmission time is provided which is made to shiftthe respective transmission time within the measurement periods (100) byan offset; and wherein the offsets form a distribution (56, 60) whosecenter of mass forms a desired transmission time and which can beselected with a time resolution which is better than both the samplingperiod and the difference period, in particular with a time resolutionof fewer than ten picoseconds or even less than one picosecond.
 9. Asensor (10) in accordance with claim 8, wherein the distribution of theoffsets is unimodal, is in particular preset in accordance with atriangular, parabolic or Gaussian function, and wherein a memory isprovided in which a table for the unit (40) for the fine setting of atransmission time is stored which holds an associated offsetdistribution for a plurality of time increments, in particular onerespective offset distribution for time increments distributed evenlyover a sampling period and/or over the time interval between twodiscrete times.
 10. A sensor (10) in accordance with claim 1, whereinthe evaluation unit (18) is made to check with reference to a transitioncondition of the received light signal whether the light signal isreceived at an observation time; and wherein a regulator (44) isprovided which is made to adjust the transmission time or the desiredtransmission time by means of an additional transmission delay time suchthat the transition condition is satisfied at the observation time. 11.A sensor (10) in accordance with claim 10, wherein a regulation timeinterval (101) within which the regulator (44) can check the transitioncondition and can adjust the transmission delay only corresponds to apartial region of a measurement zone of the sensor (10); and wherein achange of position monitoring unit (44) is provided to checkperiodically the time at which the received signal is received and, ifthis time is outside the regulation time interval (101), to preset a newregulation time interval (101) for the regulator (44); wherein thechange of position monitoring unit (44) has an agent, that is a processactive constantly or at regular allocated time slots and independent ofthe regulator (44); and wherein the agent (44) has the goal of locatinga regulation time interval (101) and of presetting it for the regulator(44) in which the light signal is received.
 12. A method for themeasurement of distances or distance changes in accordance with thelight transit time principle in which a light signal is transmitted andthe remitted or reflected light signal is received, the transmissiontime for the light signal is fixed relative to a reference time and thereceived light signal is digitized to a sampling pattern (108) having asampling period to determine the reception time of the light signal, andwherein the transmission time relative to the scanning pattern (108) isdetermined by means of a time base unit (38) with a time precision belowthe scanning period, wherein the time base unit (38) is made to derivethe time shift from a first time signal having a first frequency (f1)and a second time signal having a second frequency (f2) different fromthe first frequency (f1) and to provide any desired time shifts with atime resolution given by the difference period belonging to the firstand second frequencies (f1, f2).
 13. A method in accordance with claim12, wherein the first frequency (f1) and the second frequency (f2) isderived with a first divider and a second divider from a master clock(50) which also determines the reference time, wherein the first dividerand the second divider are selected such that a difference period ariseswhich is as small as possible in the range of some hundreds, some tensof picoseconds or a few picoseconds, wherein the first frequency (f1)and the second frequency (f2) is synchronized regularly to the masterclock (50).
 14. A method in accordance with claim 12, wherein thecomplete periods of the first or second frequencies (f1, f2) arecounted, and wherein the time shift is generated as the time intervalbetween the nth period of the first frequency (f1) and the mth period ofthe second frequency (f2).
 15. A method in accordance with claim 12,wherein the time shift is extended by periods of the first frequency(f1), of the second frequency (f2) or of the master clock (50).